This invention relates to a charge control agent useful for controlling triboelectricity of a toner for developing an electrostatic latent image. The present invention is also directed to a toner for use in developing an electrostatic latent image in an image forming method such as electrophotography and electrostatic printing.
Methods for developing an electrostatic latent image to form a toner image include a method using a one-component type developer composed only of a toner and a method using a two-component type developer composed of a toner and a carrier which are charged in opposite polarity. The one-component type method may be, for example, a powder cloud developing method in which toner particles are sprayed over an electrostatic latent image-bearing surface, a contact developing method (touch down method) in which a latent image-bearing surface is directly contacted with a bed of toner, or a dielectric developing method in which a latent image-bearing surface is contacted with a magnetic, electrically conductive toner. The two-component type method may be, for example, a magnet brush method using iron powder as a carrier, a cascade method in which glass beads are used as a carrier or a fur brush method in which a fiber brush is used as a carrier.
In the above developing systems, fine particles each composed of a matrix of a synthetic or natural binder resin, such as a polystyrene resin, and a colorant, such as carbon black, dispersed in the matrix are generally used as a toner. Such a toner is finely divided into a particle size of, for example, 1-30 xcexcm. In the case of a magnetic toner, a magnetic material such as magnetite is incorporated into the above toner.
Toner to be used for developing an electrostatic latent image is positively or negatively charged according to the polarity of the electrostatic latent image to be developed. Because the amount of charges generated by using the triboelectricity of the binder resin is small, a charge control agent is generally incorporated into the toner to obtain clear images.
Known charge controlling agents include metal complexes of monoazo dyes; humic or nitrohumic acid or a salt thereof; metal complexes (e.g. Ni, Co, Cr, and Fe metal complexes) of aromatic hydroxycarboxylic or dicarboxylic acids such as salicylic acid, naphthoic acid and dicarboxylic acid; sultonated copper phthalocyanine pigments; nitrified or halogenated styrene oligomers; chlorinated paraffins; and melamine resins. The known charge controlling agents, however, suffer from one or more defects such as non-uniformity in triboelectricity and susceptibility to decomposition during kneading.
JP-A-HO1-306861 discloses the use of an organic boron compound as a charge control agent. JP-A-S61-3149 discloses the use of a bisphenol compound as a charge control agent. These charge control agents are, however, hygroscopic in nature and are apt to cause a change in their triboelectricity. Additionally, they do not have sufficiently high saturation triboelectricity.
Incidentally, a polyester resin or an epoxy resin is now increasingly used as a binder resin for toners because of their advantages that these resins do not adversely affect the inherent color of colorants thereof, that these resins have good resistance to tackiness to polyvinyl chloride mats and that these resins have good preservation stability and yet permit low temperature fixation. When these binder resins are used in conjunction with the above known charge control agents, sufficiently high triboelectricity cannot be obtained or the triboelectricity gradually reduces even when the initial triboelectricity is high, so that problems of fatigue or toner dispersion are apt to be caused. Probably, the problems are attributed to the presence of xe2x80x94COOH and xe2x80x94OH functional groups in these resins which groups would prevent maintenance of stable triboelectricity of the charge control agents.
(1) Freedom of Hot Offset
Conventionally, an oil is applied to a heated roll to prevent xe2x80x9chot offsetxe2x80x9d which is a phenomenon occurring in fixation of toner image on paper with a heated roll and refers to deposition of fused toner onto the heated roll. The use of the oil requires the installation of an oil tank and hinders compactness of the apparatus. Recent trend is toward incorporation of a wax into the toner for improving releasability. The use of wax, however, reduces the fluidity of the toner particles and, hence, it becomes difficult to obtain desired amount of charge in a moment by friction. Since the frictional contact time in the case of a one-component type toner is much shorter than that for a two-component type toner, there is a great demand for a charge control agent having excellent triboelectricity.
Additionally, there is an increasing demand for white or colorless charge control agents which can be suitable used for color toners.
It is, therefore, an object of the present invention to provide a charge control agent which, when incorporated into toner particles, can provide a triboelectricity suitable for a given image developing system upon frictional contact between toner particles, between toner and carrier, between toner and a developer bearing member (such as a developer sleeve) and between toner and a developer adjusting member (such as a doctor blade) and which can maintain the triboelectricity in a stable manner.
Another object of the present invention is to provide a charge control agent of the above-mentioned type which can afford suitable triboelectricity in a moment irrespective of presence or absence of a releasing agent and of difference is the developing system.
It is a further object of the present invention to provide a charge control agent which can be suitably used for both one-component type and two-component type developers.
It is yet a further object of the present invention to provide a charge control agent which can provide stable triboelectricity even placed in different environmental conditions.
It is yet a still further object of the present invention to provide a charge control agent which do not adversely affect the color tone of a colorant conjointly contained in toner.
It is a further object of the present invention to provide a charge control agent which has a good heat resistance and can withstand kneading conditions for the preparation of toners.
It is a further object of the present invention to provide a charge control agent which has poor affinity with moisture.
It is a further object of the present invention to provide a charge control agent which is cheap and can reduce the cost of the toner.
It is an important object of the present invention to provide a toner containing the above charge control agent and exhibiting excellent properties attributed to the charge control agent.
In accomplishing the foregoing objects, there is provided in accordance with one aspect of the present invention a charge control agent comprising a barium salt of a sulfoisophthalic acid compound represented by the following formula (1): 
wherein R is a hydrogen atom or a hydrocarbyl group having 1 to 8 carbon atoms.
In another aspect, the present invention provides a charge control agent as described above, which further comprises a metal salt of a salicylic acid compound, wherein the metal of a salicylic acid compound is other than a barium salt.
The present invention also provides a toner for developing an electrostatic latent image, comprising a binder resin, a colorant and a charge control agent as defined above.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.
The charge control agent of the present invention comprises a barium salt of a sulfoisophthalic acid compound represented by the following formula (1): 
wherein R is a hydrogen atom or a hydrocarbyl group having 1 to 8 carbon atoms.
In the above formula (1), the hydrocarbyl group R may be a linear aliphatic hydrocarbon group such as methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, n-pentyl, n-hexyl, n-heptyl or n-octyl, a cyclic aliphatic hydrocarbon group such as cyclohexyl, an aryl group such as phenyl or an aralkyl group such as benzyl. For reasons of stability in triboelectricity, a dimethyl ester represented by the following formula (2): 
is preferably used as the barium salt of a sulfoisophthalic acid compound.
Specific examples of the barium salt of a sulfoisophthalic acid compound of the above formula (1) include 5-sulfoisophthalic acid barium salt (compound of the above formula (1) in which R is a hydrogen atom), dimethyl 5-sulfoisophthalate barium salt (compound of the above formula (2)), diethyl 5-sulfoisophthalate barium salt, dipropyl 5-sulfoisophthalate barium salt, diisopropyl 5-sulfoisophthalate barium salt, dibutyl 5-sulfoisophthalate barium salt, diisobutyl 5-sulfoisophthalate barium salt, di-t-butyl 5-sulfoisophthalate barium salt, dipentyl 5-sulfoisophthalate barium salt, di-1-methylbutyl 5-sulfoisophthalate barium salt, di-1-ethylpropyl 5-sulfoisophthalate barium salt, diisoamyl 5-sulfoisophthalate barium salt, di-2,2-dimethylpropyl 5-sulfoisophthalate barium salt, dihexyl 5-sulfoisophthalate barium salt, di-1-methylpentyl 5-sulfoisophthalate barium salt, di-1-ethylbutyl 5-sulfoisophthalate barium salt, di-1,3-dimethylbutyl 5-sulfoisophthalate barium salt, di-1,1,2-trimethylpropyl 5-sulfoisophthalate barium salt, 1,2,2-trimethylpropyl 5-sulfoisophthalate barium salt, diheptyl 5-sulfoisophthalate barium salt, di-1-methylhexyl 5-sulfoisophthalate barium salt, di-1,1-dimethylpentyl 5-sulfoisophthalate barium salt, bis-diisopropylmethyl 5-sulfoisophthalate barium salt, dioctyl 5-sulfoisophthalate barium salt, di-1-methylheptyl 5-sulfoisophthalate barium salt, di-1-ethylhexyl 5-sulfoisophthalate barium salt, di-1-ethyl-3-methylpentyl 5-sulfoisophthalate barium salt and di-2-ethylhexyl 5-sulfoisophthalate barium salt.
It is preferred that the barium salt of a sulfoisophthalic acid compound represented by the following formula (1) be used in conjunction with a metal salt of a salicylic acid compound, wherein the metal of a salicylic acid compound is other than a barium salt, for reasons of stability in triboelectricity, reduction of costs and, when applied to a wax-containing toner, improvement of dispersibility of the wax in the toner. Such an improvement in wax dispersibility is advantageous because deposition of the wax to carrier particles, a developer bearing member (e.g. developing sleeve) and/or a developer regulating member (e.g. doctor blade) is able to be prevented.
The metal salt of a salicylic acid compound is preferably a compound of the following formula (4): 
in which R2, R3 and R4 are independently selected from the group consisting of a hydrogen atom, a halogen atom or a hydrocarbyl group having 1 to 6 carbon atoms. Examples of the hydrocarbyl group may include an aliphatic group, alicyclic group, an aryl group and an aralkyl group.
The metal of the metal salt of a salicylic acid compound may be, for example, Al, Zn, Ti, Sr, B, Si, Ni, Fe, Cr and Zr. For reasons of high triboelectricity and whiteness of the color, zinc salts, chromium salts and zirconium salts are preferred. The use of a zinc salt of a salicylic acid compound is particularly preferred. The most preferred salt of a salicylic acid compound is represented by the following formula (3): 
The metal salt of a salicylic acid compound is preferably used in such an amount that the weight ratio of the barium salt of a sulfoisophthalic acid ester to the metal salt of a salicylic acid compound is greater than 1 (namely greater than 50:50), more preferably 61:49 to 98:2, most preferably 85:15 to 95:5, for reasons of good stability such as resistance to high temperature and high humidity.
The charge control agent is suitably applied to a toner for developing an electrostatic latent image. Thus, in one aspect of the present invention, there is provided a toner including a binder resin, a colorant and the above charge control agent. The toner may be used for both a one-component type developer and a two-component developer and may include any ingredients conventionally known for the preparation of toners. Description will be next made of typical ingredients of the toner.
Illustrative of suitable binder resins are homopolymers or copolymers of two or more of the following monomers: styrene or its homologues such as chlorostyrene, vinyltoluene, vinyl chloride, vinyl acetate, vinyl propionate, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, dodecyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, 2-chloroethyl (meth)acrylate, (meth)acrylonitrile, (meth)crylamide, (meth)acrylic acid, vinyl ethyl ether, vinyl methyl ether, vinyl isobutyl ether, vinyl methyl ketone, N-vinyl pyrrolidone, N-vinylpyridine and butadiene. Other resins such as polyester resins, polyurethane resins, polyol resins, polyamide resins, epoxy resins, rosins, modified rosins, terpene resins, phenolic resins, hydrogenated petroleum resins, ionomer resins, silicone resins, ketone resins, and xylene resins may also be used. Mixtures of two or more of the above homopolymers, copolymers and resins may also be used.
Polyester resins, epoxy resins and polyol resins obtained from epoxy resins have an advantage over conventionally widely used styrene-acrylate copolymer resins with respect to preservability and fixing properties but are disadvantageous with respect to charging amount. Since the charge control agent according to the present invention has high triboelectricity, polyester resins, epoxy resins or polyol resins can be suitably used as a binder resin for the toner of the present invention. The toner containing these resins can exhibit the above-mentioned advantages of these resins with respect to preservability and fixing properties as well as excellent frictional electrification properties.
Any known colorant may be used for the purpose of the invention. The colorant may be, for example, a black colorant such as carbon black, a nigrosine dye, iron black; a yellow colorant such as Naphthol Yellow S, HANSA YELLOW (10G, 5G, G, GR, A, RN and R), cadmium yellow, Chinese yellow, chrome yellow, yellow iron oxide, titanium yellow, Polyazo Yellow, Oil Yellow, Pigment Yellow L, Benzidine Yellow (G and GR), Permanent Yellow NCG, Vulcan Fast Yellow, Tartradine Lake, Quinoline Yellow Lake, Anthrazan Yellow BGL, Isoindolinone Yellow; a magenta colorant such as red iron oxide, red lead, lead vermilion, cadmium red, cadmium mercury red, antimony vermilion, Permanent Red 4R, Para Red, Fisay Red, parachloro-orthonitroaniline red, LITHOL FAST SCARLET, Brilliant Fast Scarlet, Brilliant Carmine BS, Permanent Red (F2R, F4R, FRL, FRLL and F4RH), Fast Scarlet VD, Vulcan Fast Lubin B, Brilliant Scarlet G, LITHOL LUBIN GX, Permanent Red F5R, Brilliant Carmine 6B, Pigment Scarlet 3B, Bordeaux 5B, Toluidine Maroon, Permanent Bordeaux F2K, Hello Bordeaux BL, Bordeaux 10B, Bon Maroon Light, Bon Maroon Medium, Eosin Lake, Rhodamine Lake B, Rhodamine Lake Y, Alizarmne Lake, Thioindigo Red B, Thioindigo Maroon, Oil Red, Quinacridon Red, Pyrazolon Red, Polyazo Red, chrome vermilion; Benzidine Orange, Perinon Orange, Oil Orange; a cyan colorant such as Cobalt Blue, Selulian Blue, Alkali Blue Lake, Peacock Blue Lake, Victoria Blue Lake, non-metal Phthalocyanine Blue, Phthalocyanine Blue, Fast Sky Blue, Indanthrene Blue (RS and BC), Indigo, Iron Blue, Ultramarine Blue, Anthraquinone Blue, Fast Violet B, Methyl Violet Lake, Cobalt Violet, Manganese Violet, Dioxan Violet, Anthraquinone Violet; Chrome Green, Zinc Green, chromium oxide, Pylidian, Emerald Green, Pigment Green B, Naphthol Green B, Green Gold, Acid Green Lake, Marakite Green Lake, Phthalocyanine Green, Anthraquinone Green; titanium oxide, Chinease White or Lithopone. These colorants may be used by themselves or in combination with two or more. The colorant is generally used in an amount of 0.1 to 50 parts by weight per 100 parts by weight of the binder resin.
The toner of the present invention may contain another charge control agent in addition to the above-described charge control agent for enhancing the chargeability thereof, if desired. Any charge control agent generally used in the field of toners for use in developing an electrostatic latent image may be used for the purpose of the present invention. Examples of such additional charge control agents include a nigrosine dye, a triphenylmethane dye, a chromium-containing metal complex dyer a molybdic acid chelate pigment, a rhodamine dye, an alkoxyamine, a quaternary ammonium salt including a fluorine-modified quaternary ammonium salt, alkylamide, phosphorus and a phosphorus-containing compound, tungsten and a tungsten-containing compound and a fluorine-containing activator material.
The amount of the charge control agent (including the additional charge control agent) used for enhancing the chargeability use in the color toner may be determined in light of the amount of the charge control agent, the kind of binder resin to be employed, the presence or absence of additives, and the preparation method of the toner including the method of dispersing the composition of the toner. It is preferable that the amount of the additional charge controlling agent be in the range of 0.1 to 10 parts by weight, and more preferably in the range of 2 to 5 parts by weight, per 100 parts by weight of the binder resin. By the addition of the additional charge controlling agent in such an amount, sufficient chargeability for use in practice can be imparted to the toner. Further, electrostatic attraction of the toner to a developing roller can be prevented, so that the decrease of fluidity of the developer and the decrease of image density can be prevented.
A releasing agent may be suitably incorporated into the toner. In general, a releasing agent tends to adversely affect the charging characteristics of a developer. For example, during a long period of use, the releasing agent is apt to deposit on surfaces of carriers to reduce the triboelectricity of the developer. In the case of the present invention, the use of the charge control agent having good charging characteristics can inhibit a reduction of charging amount which might be otherwise caused by the releasing agent. The releasing agent also has a problem because a toner containing the releasing agent is slow to establish the desired charging amount. Such a problem may be also overcome by the use of the charge control agent of the present invention.
Any known releasing agent may be suitably used for the purpose of the present invention. Wax is generally used as a releasing agent. Examples of the wax include low molecular weight polyolefin wax such as low molecular weight polyethylene wax and low molecular weight polypropylene wax; synthetic hydrocarbon wax such as Fischer-Tropsh wax; natural wax such as carnauba wax, candelilla wax, rice wax, montan wax; petroleum waxe such as paraffin wax and microcrystalline wax; higher fatty acids such as stearic acid, palmitic acid and millystyric acid; metal salts or amides of higher fatty acids; and modified waxes of the above waxes. These waxes may be used singly or in combination of two or more thereof. It is preferred that the wax have a melting point in the range of 70xc2x0 C. to 125xc2x0 C. for reasons of satisfactory transferability, duration and releasability. The releasing agent is generally used in an amount of 1 to 15% by weight based on the weight of the toner.
The toner of the present invention may contain a magnetic material to provide a magnetic toner. The magnetic material may be, for example, iron oxide (e.g. magnetite, ferrite or hematite), metallic cobalt or nickel, an alloy of iron, cobalt and/or nickel with one or more metals such as aluminum, copper, lead, magnesium, tin, zinc, antimony, beryllium, bismuth, cadmium, calcium, manganese, selenium, titanium, tungsten and vanadium. Above all, use of magnetite is preferred. The magnetic particles preferably have an average particle diameter of 0.1 to 2 xcexcm and are preferably used in an amount of 20 to 200 parts by weight per 100 parts by weight of the binder resin of the toner.
In addition, the toner of the present invention may contain an external additive such as silica powder, metal salts of fatty acids (such as zinc stearate and aluminum stearate), metal oxides (titanium oxide, aluminum oxide, tin oxide and antimony oxide) and fluoropolymers. Especially, hydrophobized silica powder, hydrophobized titania powder and hydrophobized alumina powder are preferred. Especially, as the hydrophobic-treated titanium oxide powder, crystalline titanium oxides including anatase-type titanium oxides and rutile-type titanium oxides, and noncrystalline titanium oxide can be used.
A hydrophobic-treated powder of silica, titania or alumina can be obtained by treating hydrophilic particles thereof with a silane coupling agent such as methyltrimethoxysilane, methyltriethoxysilane, and octyltrimethoxysilane.
The toner of the present invention may be prepared by any conventionally known method. The toner preferably has a volume particle diameter of 4-10 xcexcm for reasons of prevention of background stains and good fluidity. Granulation may be performed by, for example, pulverization, polymerization in an aqueous medium or a combination thereof.
The toner of the present invention including the above ingredients can be used in combination with a carrier as a two-component developer or alone as a one-component developer.
When the toner is used as a two-component developer, any conventionally known carrier such as iron powder, ferrite particles, and glass beads can be used. These carrier base particles may be coated with a resin. In this case, any conventionally known resin can be used. Specific examples of the resin include acrylic resins, polycarbon fluoride, polyvinyl chloride, polyvinylidene chloride, phenol resins, polyvinyl acetal and silicon resins. The carrier base particles generally has an average particle diameter of 10 to 1000 xcexcm, preferably 30 to 500 xcexcm. In general, the toner is mixed with the carrier in an amount of 0.5 to 6.0 parts by weight per 100 parts by weight of the carrier.
The charge control agent according to the present invention may be also used as a surface material which is desired to charge toner particles by frictional contact therewith. In a system for developing an electrostatic latent image using a single component developer, the toner has a smaller chance for frictional electronification as compared with a two-component developer which contains a carrier. Thus, so the toner of the single component developer can be charged sufficiently within a short period of time, the charge control agent may be incorporated into a developer bearing member (developing roller or sleeve), a developer regulating member (doctor blade) for adjusting the thickness of the developer layer on the developer bearing member or a toner feed roller.
The following examples will further illustrate the present invention. Parts are by weight.